Fix issue JuliaPackaging#121 (JuliaPackaging#125)

Fix issue JuliaPackaging#121

Change `detect_libcxx_abi()` to return `:gcc_any` and just throw a warning instead of an error when it can't find symbols properly.

Author: Rogerluo

src/PlatformNames.jl

@@ -619,7 +619,8 @@ function detect_libstdcxx_abi()
 
     # Full list available here: https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html
     if max_version < v"3.4.18"
-        error("libstdc++ ABI version '$max_version' too small; please build Julia with GCC >= 4.8.5")
+        @warn "Cannot make sense of autodetected libstdc++ ABI version ('$max_version')"
+        return :gcc_any
     elseif max_version < v"3.4.23"
         # If we aren't up to 7.1.0, then we fall all the way back to 4.8.5
         return :gcc4